1. Field of the Disclosure
Embodiments disclosed herein relate generally to riser tensioners used on floating drilling vessels. Particularly, embodiments disclosed herein relate to apparatus and methods to secure a riser tensioner when not in use.
2. Background Art
Conventional offshore drilling for oil and natural gas from floating vessels typically requires a drilling riser, which is an assembly of jointed pipes that convey fluids from the drilling vessel down to a wellhead located on the seabed. Drilling risers must be kept in tension; otherwise, they may buckle under their own weight and the weight of the fluids they contain. Tension is typically maintained by a combination of floatation equipment attached to the drilling riser, and by a device on the drilling vessel called a riser tensioner.
Early drilling riser tensioners included wire ropes, one end of which was attached to the drilling riser, and the other end reeved over sheaves attached to large hydraulic cylinders that are attached to a drilling vessel. When the vessel heaved, the tension in the wire ropes, and thus in the drilling riser, was maintained by the hydraulic cylinders acting on the sheaves over which the wire ropes are reeved.
As offshore drilling has moved to deeper waters and lengths of drilling risers have increased, the required tension on the drilling riser has also increased, which has required larger diameter wire ropes. Large rope diameters, combined with repetitive stress cycles and the corrosive marine environment, may lead to much shorter service life for the wire ropes. Consequently, newer deepwater floating drilling rigs are equipped with direct-acting riser tensioners as disclosed in U.S. Pat. No. 4,379,657, issued to Widiner, in which a plurality of hydraulic cylinders are attached directly between the top of the drilling riser and the bottom of the floating drilling vessel.
Direct-acting riser tensioners may be advantageous over wire rope tensioners because the entire tensioner assembly may be moved away from the well-center and stowed. Certain operations, such as running or pulling the riser, or during transit of the vessel from one drilling location to another, may require stowing of the riser tensioner. Stowing the direct-acting riser tensioner typically involves moving it along a pair of tracks located in the moon pool area. One version of this track system for displacing a direct-acting riser tensioner system away from well-center is called the “N-Line Trip Saver” and is available from National Oilwell Varco, of Houston, Tex. In this system, the hydraulic cylinders of the riser tensioner are attached to a trolley that is propelled along a track by a hydraulically powered rack-and-pinion system.
However, when the direct-acting riser tensioner assembly is in the stowed position, it may be free to move about within the moonpool and can potentially cause damage to the vessel, to itself, or to associated equipment during periods of rough seas. Conventionally, the bottom of the riser tensioner assembly may be restrained by wire rope and shackles, but this approach is time-consuming, requires large diameter wire rope to handle the large loads involved, and requires a worker be suspended over open water in the moon pool to handle and attach the wires. If the seas are not calm, this operation may be hazardous.
Accordingly, there exists a need for apparatus and methods to restrain a direct-acting hydraulic riser tensioner assembly which will quickly and safely lock the riser tensioner in the stowed position.